What are the advantages of the inline function?

The reason why the inline function is fast is because Theoretically, unlike normal functions, the compiler changes the invocation part of the inline function to the content of the function Because I don't push/pop on the call stack. (However, sometimes the compiler doesn't handle it inline, so the inline function is not always faster.)

As you said, it doesn't make a huge difference these days, but it's a little faster to use online if you do a lot of calculations as below.

inline int aplusb_pow2(int a, int b) {
  return (a + b)*(a + b) ;
}

for(int a = 0; a < 900000; ++a)
    for(int b = 0; b < 900000; ++b)
        aplusb_pow2(a, b);